Transformers are used in many different types of power distribution systems, such as in switched voltage converters. An example of a switched voltage converter utilizing a transformer is the diagonal half-bridge flyback converter of FIG. 1. In a first portion of a switching cycle, both transistors 102 and 104 are ON and store energy in the magnetic field of transformer 106. All the diodes are OFF, i.e., reverse-biased. In a second (flyback) portion of a switching cycle, the energy previously stored in the transformer magnetic field is released to output capacitor 108 via output diode 110. Any excess energy will be returned to input capacitor 112 via input diodes 114 and 116, which also limits the voltage stress on switching transistors 102 and 104. The duty cycle depends on the transformer turn ratio (i.e. voltage conversion ratio). Controller 118 adjusts the switching frequency to regulate the amount of energy provided to load 120, so that the sensed voltage VS is close to reference voltage Vref. For a small load, the switching frequency is high. For a large load, the switching frequency is low. The coupling factor between the input and output windings of transformer 106 determines how much of the stored magnetic energy is released to the output in the second (flyback) portion of switching cycle. Low coupling factor results in poor efficiency.
The flyback converter of FIG. 1 is just one example of a switched voltage converter making use of a transformer. In many applications requiring a DC-to-DC converter, such as portable systems utilizing microprocessors, switched voltage converters may be more desirable than other types of voltage converters or regulators, such as linear voltage regulators, because they can be made more efficient. In a linear voltage regulator, the power conversion efficiency is always less than VS/VD, whereas in a switching converter, the efficiency is typically 80-95%.
Transformers find applications in power distribution systems other than the flyback converter, which is just one example. There are advantages to integrating a power distribution system on the same die as the circuits that are powered by the power distribution system. For example, as processor technology scales to smaller dimensions, supply voltages to circuits within a processor will also scale to smaller values. But for many processors, power consumption has also been increasing as technology progresses. Using an off-die voltage converter to provide a small supply voltage to a processor with a large power consumption leads to a large total electrical current being supplied to the processor. This can increase the electrical current per pin, or the total number of pins needed. Also, an increase in supply current can lead to an increase in resistive as well as inductive voltage drop across various off-die and on-die interconnects, and to a higher cost for decoupling capacitors. Integrating the voltage converter onto the die would mitigate these problems because a higher input voltage with lower current could be provided to the die by an off-die power supply, and the reduction of the higher input voltage to lower, regulated voltages could be done on the die closer to the circuits that require the regulated voltages.